1. Field of the Invention
This invention relates to laser induced dry chemical etching and, more particularly, to a method and apparatus for indicating when a pulsed etching laser, operating in the presence of a reactive gas, has etched through a layer of a first material superposed on a second material and for controlling the etching laser to terminate the etching operation in response to detection of this end point.
2. Brief Description of the Prior Art
Laser induced dry chemical etching, as is shown, for example, in U.S. Pat. No. 4,490,210 issued to Chen et al and U.S. Pat. No. 4,490,211 issued to Chen et al, both assigned to the assignee of this application, is used to etch metallized substrates in the presence of a reactive gas. The substrate is exposed to a selected gas, such as a halogen gas, which reacts spontaneously with the metal forming a solid reaction product with the metal by partial consumption of the metal. A pulsed beam of radiation of a wavelength suitable for absorption by the reaction product and/or the metal thereunder is applied in a desired pattern to vaporize the reaction product and thereby selectively etch the metal.
It is often useful to apply this technique to a substrate having a first material superposed on a second material to remove the first material selectively to expose an area of the second material. When, for example, a chromium-copper-chromium substrate is prepared for soldering, the top chromium layer must be removed without damage to the underlying copper layer. However, it has been difficult to determine accurately when the removal of the first material has reached an end point so that the etching beam may be shut off without undue damage to the layer of second material. The problem has been particularly acute with respect to the chromium-copper-chromium substrate example, because the exposed copper etches several orders of magnitude faster than the chromium.
There have been a number of suggestions in the prior art for end point detection while etching specific layers on thin film devices. U.S. Pat. No. 4,394,237 issued to Donnelly et al teaches the use of laser induced fluorescence for end point detection in connection with plasma reaction etching employing essentially continuously applied (including R.F.) plasma reactions. The signals generated with such reaction ion etching tend to be small (see R. Walkup et al, Appl. Phys. Lett., Vol. 45, p. 372, 1984), often corresponding to about 10.sup.8 diatomics/cm..sup.3. As a result, approximately 0.1 second of signal averaging may be required before the reaction can be stopped. Donnelly et al do not teach how laser induced fluorescence may be applied to laser etching.
Bennett et al application Ser. No. 878,144, filed June 25, 1986, assigned to the same assignee as the present application, also teaches using laser induced fluorescence to detect and control the reactive ion etch-through of a given layer in a wafer. A large change in the concentration of a selected minor species from the wafer in the etching plasma is detected for end point detection. When the large change in the selected minor species concentration is detected, the RF electrodes for the reactor are automatically de-energized. Bennett et al thus do not teach the application of laser induced fluorescence to laser induced dry chemical etching.
U.S. Pat. No. 4,198,261 issued to Busta et al also discloses a technique for detecting the end point of a plasma etching process. A laser probe is used to generate an interference pattern due to reflected and refracted light waves in the surface layer. Laser-induced fluorescence is not used.
Another end point detection technique is shown in U.S. Pat. No. 4,393,311 issued to Feldman et al. A surface is exposed to a beam of probe radiation and the infrared, visible or UV radiation emitted by excited particles desorbed from the surface is detected. The self-luminosity depended upon by Feldman et al would be too low to be useful for detecting an end point in the case of a chromium-copper laser etching using chlorine gas.